1. Field of the Invention
This invention relates to a top plate for cooking device having an electromagnetic-induction heating unit, and optionally having an infrared heating unit.
2. Prior Art
As heating units used in electrical cooking devices, there are known a infrared heating unit such as a radiant heater and a halogen heater, and an electromagnetic-induction heating unit using an induction heater.
A cooking device having the infrared heating unit has a top plate. The top plate has usually been made of a dark-colored crystallized glass plate which shades or cut off the visible light but transmits the infrared light. Shading of the visible light is for keeping the internal structure of the device non-operated out of sight as well as for attenuating the strong radiation including the visible light component from the halogen heater so as to decrease the brightness. The red heat of the heater unit operated can be seen through the dark-colored top plate, by which the operation of the heating device can be identified.
Another cooking device having the electromagnetic-induction heating unit does not generate the visible light component. Therefore, it is impossible to see whether the device is operating or not. In order to resolve the problem, the cooking device of the electromagnetic induction type is provided with a power indicator comprising, of example, light emission diodes (LEDs) for providing visible indication of a heating power which the device is generating. The power indicator is, in a recent leading design, disposed adjacent the electromagnetic induction heating unit and can, therefore, be seen through the top plate, as disclosed in JP 3-114182 A, although there is another design where it is mounted on an outer surface of a side wall of the device. However, the light from LEDs is weak so that it cannot be seen through the dark-colored top plate. Therefore, a light-transparent crystallized glass plate is mainly used for the top plate in the cooking device having the electromagnetic induction-type heating unit of the leading design described above.
In use of the light-transparent top plate, the internal structure such as the heating unit can be seen through the top plate. It is desired to screen the internal structure because of ornamental reasons.
A light-transparent crystallized glass plate, per se, has a smooth, flat and glossy surface, which is suitable for a top surface of the top plate. On the other hand, any ornamental coating is desired on the top surface for indicating the portions heated by the heater unit or units and for printing description how to use and/or warning phrases or sentences thereon.
It may be possible to use the ornamental coating as shading the internal structure. However, the ornamental coating is easily subjected to cracks caused due to thermal differences between different portions thereof. In order to avoid the cracks, the ornamental film may be made as a porous layer. However, the porous layer is not good as an ornamental film and is insufficient for printing the description. The porous layer is further worn by contact and friction with cooking wares such as pan, pot, dish and others.
Therefore, it is an object of this invention to provide a top plate for a cooking device having an electromagnetic heating unit which has a smooth, flat and glossy top surface and can shade the internal structure of the device.
According to this invention, a top plate in a cooking device having an electromagnetic-induction heating unit comprises:
a light-transparent crystallized glass plate with a top surface, on which foods are heated, and with a bottom surface confronting the electromagnetic-induction heating unit;
an ornamental film entirely or partially coated on said top surface, said ornamental film being a dense layer comprising a first inorganic pigment, and;
a light-shading film entirely or partially coated on said bottom surface, said light-shading film being a porous layer comprising a second inorganic pigment.
According to an embodiment, the dense layer comprises a first inorganic pigment powder and glass, while the porous layer comprises a second inorganic pigment powder and glass.
The dense layer is higher than the porous layer in the glass percentage content.
According to another embodiment, the top plate has a heated portion heated by the electromagnetic-induction heating unit, and the top plate further comprises a heat resistant resin layer formed on at least one area of the light-shading film corresponding to the heated portion.
The heat-resistant resin layer may comprise one or more selected from a group of polyimide resin, polyamide resin, fluorine-contained polymer, and silicone resin.
According to another aspect of this invention, the top plate is provided to be used in a cooking device having an infrared heating unit in addition to the electromagnetic-induction heating unit. The top plate has a first heated portion heated by the electromagnetic-induction heating unit and a second heated portion heated by said infrared heating unit.
According to another embodiment, the light-shading film is lower in the density at the second heated portion than that at the first heated portion.
According to another embodiment, the density of the light-shading film at the second heated portion is 30-80% of that at the first heated portion.
According to another embodiment, the light-shading film is a lot of apertures at the second heated portion.
Each of the apertures preferably has 0.05-5 mm diameter.
The number of apertures is preferably 5-500 per 1 cm2.
According to another embodiment, the light-shading film is smaller in the thickness at the second heated portion than that at the first heated portion.
In a preferred embodiment, the thickness of the light-shading film at the second heated portion is 10-50% of that at the first heated portion.
According to a further different embodiment, the light-shading film is formed in a luster layer at the second heated portion.